The Arabic phrase amīr-al-, “commander of the,” entered Latin title formations. French carried the naval title into English.
The French intermediary is compressed. Alternative Latin/English spellings and the proposed reason for d are not separately ranked.
Sources
- Merriam-Webster: admiral
The title has multiple attested forms and a qualified spelling explanation.
- Online Etymology Dictionary · admiral
Meanings of the older forms on the word path.
- logeion.uchicago.edu · admirallus
Meanings of the older forms on the word path.